It is occasionally necessary to generate a positive voltage above the supply voltage VDD in integrated memory devices being supplied with a single supply voltage. Integrated memory devices include EPROM, EEPROM and Flash memory devices, for example. When a positive voltage above the supply voltage is required, voltage boosters are used to generate the voltage. A charge pump circuit may be used as a voltage booster. However, charge pump circuits present some manufacturing difficulties, especially when they operate on very low supply voltages, e.g., less than 1.8 V.
Positive charge pumps are usually formed with NMOS transistors. This construction has a disadvantage, however, in the low voltage at which the NMOS transistors can be turned on. When VGS&lt;VDD, this results in low transistor conductivity and, consequently, an inefficient charge pump that supplies a rather small current. Preferably, these NMOS transistors are not be driven with a voltage VGS&gt;VDD.
This situation is aggravated by the overdrive voltage wherein the NMOS transistor can be turned on, which is always slightly less than VDD. The overdrive voltage is the difference between the gate-source voltage drop and the threshold voltage VGS-VT. This is due to the charge being shared with parasitic capacitances which oppose an ideal boost effect.
Another contribution to a depressed boost voltage is that the waveform of the phase signals that drive the capacitors have a significant finite duration. The NMOS transistors used in positive voltage boosters are also affected by the body effect. With standard process technologies, this effect cannot be suppressed. This means that a drive voltage VGS-VT=VDD-VT may be inadequate to turn on the transistors if VT&gt;VDD. As a result, the charge pump cannot output voltages above a given value. The delivered current will be small due to the turned on transistors being able to conduct no more than a small current because of being poorly driven.
The body effect is more pronounced in the last stages of the charge pump, i.e., the stages nearest the output, because they obtain higher voltages. With triple-well process technology, wherein the bulk of the NMOS transistors can be driven at voltages other than ground voltage, the body effect can be suppressed. However, triple-well processes are expensive and not readily available.